The present invention relates to an evaporated fuel treatment device in which a canister can be improved in its capability of interiorly adsorbing evaporated fuel generated in a fuel tank.
Conventionally, a vehicle mounted with an engine, for which a highly volatile fuel (for example, gasoline or the like) is utilized, is equipped with an evaporated fuel treatment device to prevent emission into the atmosphere of evaporated fuel generated in a fuel tank.
Such an evaporated fuel treatment device, as shown in FIGS. 3A and 3B, comprises a canister 110 within which evaporated fuel generated in a fuel tank 120 is temporarily adsorbed and a purge control valve 150 which is provided in the middle of a purge passage 145 connecting the canister 110 and an inlet pipe 102 and which is opened and closed according to the operational status of an engine 101. When the purge control valve 150 is opened, the evaporated fuel adsorbed within the canister 110 is purged (introduced) into the inlet pipe 102, together with air introduced by a negative pressure generated on the side of the inlet pipe 102, to be led into cylinders. At this time, a flow of an air-fuel mixture composed of the evaporated fuel and the air to be delivered to the inlet pipe 102 is adjusted by controlling opening and closing of the purge control valve 150. In this way, combustion is realized at a desired air-fuel ratio in the engine.
In the aforementioned manner, transpiration into the atmosphere of the evaporated fuel generated in the fuel tank 120 is prevented, and fuel in the fuel tank 120 is thus consumed without being wasted.
The evaporated fuel adsorbed within the canister 110 is desorbed in such a manner that it is attracted by the negative pressure generated on the side of the inlet pipe 102 when the engine 101 is started. And then, the evaporated fuel desorbed from the canister 110 is delivered to the inlet pipe 102, together with the air introduced into the canister 110, through the purge passage 145.
However, in the aforementioned conventional structure, the negative pressure generated on the side of the inlet pipe 102 is suppressed by the purge control valve 150 provided on the purge passage 145. Also, the purge control valve 150 can be a ventilation resistance for the air-fuel mixture containing the evaporated fuel desorbed from the canister 110. Accordingly, there is a certain limitation in desorption efficiency for the evaporated fuel adsorbed within the canister 110. On the other hand, since the amount of the evaporated fuel subsequently adsorbed within the canister 110 corresponds to the amount of the evaporated fuel desorbed from the canister 110 at this time, the desorption efficiency in the canister 110 also influences adsorption efficiency therein. Consequently, the canister 110 in the aforementioned conventional structure has a certain limitation in its capability of treating the evaporated fuel. And then, a canister of which capacity is large to some degree should have been employed so far, in order to treat a fixed amount of evaporated fuel.
The present invention was made to solve the aforementioned problems. The object of the invention is to provide an evaporated fuel treatment device in which a canister can be improved in its capability, such that efficiency in treatment of evaporated fuel can be enhanced while the canister itself can be downsized.
In order to attain this object, according to a first aspect of the invention, there is provided an evaporated fuel treatment device comprising:
a canister within which evaporated fuel generated in a fuel tank is adsorbed, the canister being connected to the fuel tank via an evaporation passage;
a purge passage for connecting the canister and an inlet pipe of an internal combustion engine;
an inlet air passage for introducing air to deliver to the purge passage the evaporated fuel desorbed from the canister, the inlet air passage being connected to the canister; and
flow control means for controlling a flow of the air flowing through the inlet air passages the flow control means being disposed on the inlet air passage.
In the evaporated fuel treatment device according to the first aspect of the invention, a purge control valve conventionally provided on the purge passage is removed, and instead, the flow control means is provided on the inlet air passage. This means that the canister and the inlet pipe are connected to each other without any intervening member provided therebetween that causes, particularly, flow resistance. As a result, the canister is directly subject to a negative pressure generated on the side of the inlet pipe at the time of starting of the engine, and a great attraction force thus acts on the evaporated fuel adsorbed within the canister. Also, by restricting, by means of the flow control means, the flow of the air introduced into the canister in such a manner that easing of pressure applied to the canister is prevented, action of the negative pressure applied to the canister can be further increased. And consequently, the evaporated fuel adsorbed within the canister can be attracted more strongly to the side of the inlet pipe because of the negative pressure increased. In this manner, the amount of the evaporated fuel desorbed from the canister can be increased (in other words, the amount of the evaporated fuel remaining adsorbed within the canister can be reduced) compared to cases where an evaporated fuel treatment device having a conventional structure is employed. As a result, evaporated fuel in the amount corresponding to the amount of the evaporated fuel desorbed from the canister at this time can be subsequently introduced from the fuel tank to be re-adsorbed within the canister. That is to say, with the evaporated fuel treatment device according to the first aspect of the invention, the amount of evaporated fuel which can be adsorbed within the canister at a time (what is called an effective adsorption rate) can be increased compared to that with a conventional device, and performance of the canister can thus be enhanced. As a result, the evaporated fuel treatment device in its entirety can be improved in its capability of treating evaporated fuel. Furthermore, if the performance of the canister is enhanced, another advantage can be obtained in return; that is, the canister itself can be reduced in capacity, which is advantageous in view of space-saving.
The flow control means may be an orifice by which the inlet air passage is locally narrowed. Otherwise, it may be a valve having a simple structure and able to be opened and closed on the inlet air passage. However, once an air-fuel mixture composed of the evaporated fuel desorbed from the canister and the air introduced from the inlet air passage is delivered, via the purge passage, to the inlet pipe, an air-fuel ratio of the engine is influenced by the air-fuel mixture. Accordingly, it may be necessary that the amount of the air introduced, via the inlet air passage, into the canister be properly controlled according to the condition of an engine load.
For this purpose, according to a second aspect of the invention, there is provided the evaporated fuel treatment device wherein the flow control means is constituted by a variable throttle mechanism capable of adjusting the flow of the air flowing through the inlet air passage.
In this structure, by adjusting, by means of the variable throttle mechanism, the amount of the air introduced into the canister as well as the action of the negative pressure applied to the canister, the amount of the evaporated fuel delivered to the inlet pipe can be properly adjusted.
Furthermore, according to a third aspect of the invention, there is provided the evaporated fuel treatment device wherein the variable throttle mechanism is constituted by a proportional control valve. This proportional control valve is an electromagnetic valve which can be opened and closed with timing electrically adjusted according to the condition of the engine load or the like, and it is the same one conventionally used as the purge control valve disposed on the purge passage. In other words, it can be said that, in the structure of the evaporated fuel treatment device of the intention, position of the proportional control valve conventionally used for flow control of the intake air to be delivered together with the evaporated fuel is changed from a conventional position, that is, on the purge passage, to on the inlet air passage. As a result of changing the position of the proportional control valve in this manner, desorption efficiency in the canister can be enhanced, and at the same time, flow control of the air-fuel mixture can be achieved as well for proper engine control.
In the aforementioned structure, however, a problem may arise when the negative pressure generated on the side of the inlet pipe is increased too much, for example, in cases where the operational status of the vehicle is unusual. More specifically, if a differential between an internal pressure of the canister and the atmospheric pressure becomes too large by the action of the negative pressure, the canister itself may be deformed or damaged because of such a large pressure differential. To prevent such deformation or damage, a certain restriction needs to be placed on the largeness of the negative pressure applied to the canister.
Then, according to a fourth aspect of the invention, there is provided the evaporated fuel treatment device further comprising a mechanism for blocking up the purge passage when the negative pressure applied to the canister becomes equal to or exceeds a predetermined value.
This mechanism may be constituted in such a manner that it is mechanically closed up when the pressure differential between the internal pressure of the canister and the atmospheric pressure becomes equal to or over a fixed value, this point in time being regarded as the time when the negative pressure becomes equal to or over the predetermined value. Otherwise, a pressure sensor for detecting the internal pressure of the canister may be provided separately, and in this case, the mechanism may be constituted such that it is controlled to be closed up when the internal pressure of the canister detected by the pressure sensor becomes equal to or below a fixed value, this point in time being regarded as the time when the negative pressure becomes equal to or over the predetermined value.
By constituting the evaporated fuel treatment device in the aforementioned manner, even if the negative pressure generated on the side of the inlet pipe becomes excessive, for example, in cases where any unusual situation arises during operation of the engine, the internal pressure of the canister can be kept equal to or below a fixed value, thereby protecting the canister from being deformed or damaged.